Integrated circuit heat pipe heat spreader with through mounting holes

ABSTRACT

The apparatus is a heat pipe with superior heat transfer between the heat pipe and the heat source and heat sink. The heat pipe is held tightly against the heat source by mounting holes which penetrate the structure of the heat pipe but are sealed off from the vapor chamber because they each are located within a sealed structure such as a pillar or the solid layers of the casing surrounding the vapor chamber. Another feature of the heat pipe is the use of more highly heat conductive material for only that part of the wick in the region which contacts the heat source, so that there is superior heat conductivity in that region.

BACKGROUND OF THE INVENTION

This invention relates generally to active solid state devices, and morespecifically to a heat pipe for cooling an integrated circuit chip, withthe heat pipe designed to be held in direct contact with the integratedcircuit.

As integrated circuit chips decrease in size and increase in power, therequired heat sinks and heat spreaders have grown to be larger than thechips. Heat sinks are most effective when there is a uniform heat fluxapplied over the entire heat input surface. When a heat sink with alarge heat input surface is attached to a heat source of much smallercontact area, there is significant resistance to the flow of heat alongthe heat input surface of the heat sink to the other portions of theheat sink surface which are not in direct contact with the contact areaof the integrated circuit chip. Higher power and smaller heat sources,or heat sources which are off center from the heat sink, increase theresistance to heat flow to the balance of the heat sink. This phenomenoncan cause great differences in the effectiveness of heat transfer fromvarious parts of a heat sink. The effect of this unbalanced heattransfer is reduced performance of the integrated circuit chip anddecreased reliability due to high operating temperatures.

The brute force approach to overcoming the resistance to heat flowwithin heat sinks which are larger than the device being cooled is toincrease the size of the heat sink, increase the thickness of the heatsink surface which contacts the device to be cooled, increase the airflow which cools the heat sink, or reduce the temperature of the coolingair. However, these approaches increase weight, noise, systemcomplexity, and expense.

It would be a great advantage to have a simple, light weight heat sinkfor an integrated circuit chip which includes an essentially isothermalsurface even though only a part of the surface is in contact with thechip, and also includes a simple means for assuring intimate contactwith the integrated circuit chip to provide good heat transfer betweenthe chip and the heat sink.

SUMMARY OF THE INVENTION

The present invention is an inexpensive heat pipe heat spreader forintegrated circuit chips which is of simple, light weight construction.It is easily manufactured, requires little additional space, andprovides additional surface area for cooling the integrated circuit andfor attachment to heat transfer devices for moving the heat away fromthe integrated circuit chip to a location from which the heat can bemore easily disposed of. Furthermore, the heat pipe heat spreader isconstructed to assure precise flatness and to maximize heat transferfrom the heat source and to the heat sink, and has holes through itsbody to facilitate mounting.

The heat spreader of the present invention is a heat pipe which requiresno significant modification of the circuit board or socket because it isheld in intimate contact with the integrated circuit chip byconventional screws attached to the integrated circuit mounting board.This means that the invention uses a very minimum number of simpleparts. Furthermore, the same screws which hold the heat spreader againstthe chip can also be used to clamp a finned heat sink to the oppositesurface of the heat spreader.

The internal structure of the heat pipe is an evacuated vapor chamberwith a limited amount of liquid and includes a pattern of spacersextending between and contacting the two plates or any other boundarystructure forming the vapor chamber. The spacers prevent the plates frombowing inward, and therefore maintain the vital flat surface for contactwith the integrated circuit chip. These spacers can be solid columns,embossed depressions formed in one of the plates, or a mixture of thetwo. Porous capillary wick material also covers the inside surfaces ofthe heat pipe and has a substantial thickness surrounding the surfacesof the spacers within the heat pipe, thus forming pillars of porous wicksurrounding the supporting spacers. The wick material therefore spansthe space between the plates in multiple locations.

The spacers thus serve important purposes. They support the flat platesand prevent them from deflecting inward and distorting the plates todeform the flat surfaces which are required for good heat transfer. Thespacers also serve as critical support for the portions of the capillarywick which span the internal space between the plates. The capillarywick pillars which span the space between the plates provide a gravityindependent characteristic to the heat spreader, and the spacers aroundwhich the wick pillars are located assure that the capillary wick is notsubjected to destructive compression forces.

The spacers also make it possible to provide holes into and through thevapor chamber, an apparent inconsistency since a heat pipe vacuumchamber is supposed to be vacuum tight. This is accomplished by bondingthe spacers, if they are solid, to both plates of the heat pipe, or, ifthey are embossed in one plate, bonding the portions of the depressionswhich contact the opposite plate to that opposite plate. With the spacerbonded to one or both plates, a through hole can be formed within thespacer and it has no effect on the vacuum integrity of the heat pipevapor chamber, from which the hole is completely isolated.

An alternate embodiment of the invention provides the same provision formounting the heat pipe heat spreader with simple screws even when theheat pipe is constructed without internal spacers. This embodiment formsthe through holes in the solid boundary structure around the outsideedges of the two plates. This region of the heat pipe is by its basicfunction already sealed off from the vapor chamber by the bond betweenthe two plates, and the only additional requirement for forming athrough hole within it is that the width of the bonded region be largerthan the diameter of the hole. Clearly, with the holes located in theperipheral lips, the heat pipe boundary structure can be any shape.

Another alternate embodiment of the invention provides for improved heattransfer between the integrated circuit chip and the heat pipe heatspreader. This is accomplished by using a different capillary wickmaterial within the heat pipe at the location which is directly incontact with the chip. Instead of using the same sintered copper powderwick which is used throughout the rest of the heat pipe, the part of thewick which is on the region of the heat pipe surface which is in contactwith the chip is constructed of higher thermal conductivity sinteredpowder. Such powder can be silver, diamond, or many other materials wellknown in the art. This provides for significantly better heat transferin the most critical heat transfer area, right at the integrated circuitchip.

The present invention thereby provides a heat pipe with superior heattransfer characteristics, and the simplest of all mounting devices, justseveral standard screws.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section view of the preferred embodiment of the heatpipe of the invention with through holes through its vapor chamber andin contact with a finned heat sink.

FIG. 2 is a cross section view of an alternate embodiment of the heatpipe of the invention with through holes in the peripheral lips and in adepression in one plate.

FIG. 3 is a plan view of an internal surface the contact plate of thepreferred embodiment of the invention showing the region of thecapillary wick constructed of sintered higher heat conductivity powder.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a cross section view of the preferred embodiment of a flatplate heat pipe 10 of the invention with through holes 12 through itsvapor chamber 14 and in contact with finned heat sink 16.

Heat pipe 10 is constructed by forming a boundary structure by sealingtogether two formed plates, contact plate 18 and cover plate 20. Contactplate 18 and cover plate 20 are sealed together at their peripheral lips22 and 24 by conventional means, such as soldering or brazing, to formheat pipe 10. Heat pipe 10 is then evacuated to remove allnon-condensible gases and a suitable quantity of heat transfer fluid isplaced within it. This is the conventional method of constructing a heatpipe, and is well understood in the art of heat pipes.

The interior of heat pipe 10 is, however, constructed unconventionally.While contact plate 18 is essentially flat with the exception ofperipheral lip 24, cover plate 20 includes multiple depressions 26.Depressions 26 are formed and dimensioned so that, when contact plate 18and cover plate 20 are joined, the flat portions of depressions 26 arein contact with inner surface 28 of contact plate 18. Depressions 26thereby assure that the spacing between contact plate 18 and cover plate20 will be maintained even though pressure differentials between theinside volume of heat pipe 10 and the surrounding environment mightotherwise cause the plates to deflect toward each other.

Heat pipe 10 also includes internal sintered metal capillary wick 30which covers the entire inside surface of contact plate 18. As is wellunderstood in the art of heat pipes, a capillary wick provides themechanism by which liquid condensed at the cooler condenser of a heatpipe is transported back to the hotter evaporator where it isevaporated. The vapor produced at the evaporator then moves to thecondenser where it again condenses. The two changes of state,evaporation at the hotter locale and condensation at the cooler site,are what transport heat from the evaporator to the condenser.

In the present invention, heat pipe 10 also has capillary wick pillars32 which bridge the space between contact plate 18 and cover plate 20.Pillars 32 thereby interconnect cover plate 16 and contact plate 14 withcontinuous capillary wick. This geometry assures that, even if heat pipe10 is oriented so that cover plate 16 is lower than contact plate 14,liquid condensed upon inner surface 34 of cover plate 20 will still bein contact with capillary pillars 32. The liquid will therefore be movedback to raised surface 28 which functions as the evaporator because itis in contact with a heat generating integrated circuit (not shown).Capillary pillars 32 are wrapped around and supported by depressions 26,which prevents the structurally weaker capillary pillars 32 fromsuffering any damage.

FIG. 1 also shows frame 36 which is typically used to surround andprotect heat pipe 10. Frame 34 completely surrounds heat pipe 10 andcontacts lip 24 of contact plate 18. When heat pipe 10 is used to coolan integrated circuit chip (not shown) which is held against contactplate 18, cover plate 20 is held in intimate contact with fin plate 38,to which fins 16 are connected. The entire assembly of heat pipe 10,frame 34, and fin plate 38 is held together and contact plate 18 is heldagainst an integrated circuit chip by conventional screws 40, shown indashed lines, which are placed in holes 42 in fin plate 38 and throughholes 12 in heat pipe 10, and are threaded into the mounting plate (notshown) for the integrated circuit chip.

Holes 12 penetrate heat pipe 10 without destroying its vacuum integritybecause of their unique location. Holes 12 are located within sealedstructures such as solid columns 44, and since columns 44 are bonded tocover plate 20 at locations 46, holes 12 passing through the interior ofcolumns 44 have no affect on the interior of heat pipe 10.

The preferred embodiment of the invention has been constructed as heatpipe 10 as shown in FIG. 1. This heat pipe is approximately 3.0 inchesby 3.5 inches with a total thickness of 0.200 inch. Cover plate 20 andcontact plate 18 are constructed of OFHC copper 0.035 inch thick, anddepressions 26 span the 0.100 inch height of the internal volume of heatpipe 10. The flat portions of depressions 26 are 0.060 inch in diameter.Capillary wick 30 is constructed of sintered copper powder and averages0.040 inch thick. Columns 44 have a 0.250 inch outer diameter, and holes12 are 0.210 in diameter.

FIG. 2 is a cross section view of an alternate embodiment of the flatplate heat pipe 11 of the invention with through holes 48 located withinperipheral lips 22 and 24 of the heat pipe and hole 50 shown in anothersealed structure, one of the depressions 26. The only requirement forforming hole 50 within a depression 26 is that the bottom of depression26 must be bonded to inner surface 28 of contact plate 18 to preventloss of vacuum within the heat pipe. Of course, the region of theperipheral edges is also a sealed structure since bonding between lips22 and 24 is inherent because heat pipe 11 must be sealed at its edgesto isolate the interior from the outside atmosphere.

The only differences between heat pipe 11 of FIG. 2 and heat pipe 10 ofFIG. 1 are that finned heat sink 16 is not shown in FIG. 2, lips 22 and24 are slightly longer in FIG. 2 to accommodate holes 48, and hole 50 isshown. In fact, through holes 12 shown in FIG. 12 are also included inFIG. 2. Although it is unlikely that holes 12, holes 48, and hole 50would be used in the same assembly, manufacturing economies may make itdesirable to produce all the holes in every heat pipe so that the sameheat pipe heat spreader can be used with different configurations offinned heat sinks. The unused sets of holes have no effect on theoperation or benefits of the invention.

FIG. 3 is a plan view of the internal surface of the contact plate 18 ofheat pipe 10 of the invention showing region 31 of capillary wick 30.Region 31 is constructed of sintered silver powder. While the balance ofcapillary wick 30 is conventional sintered metal such as copper, region31 of capillary wick 30, which is on the opposite surface of contactplate 18 from the integrated circuit chip (not shown), is formed ofpowdered silver. The higher thermal conductivity of silver yieldssignificantly better heat conduction through region 31 of the wick 30,and thereby reduces the temperature difference between the integratedcircuit chip and the vapor within heat pipe 10. This reduction oftemperature difference directly affects the operation of heat pipe 10,and essentially results in a similar reduction in the operatingtemperature of the chip.

The invention thereby furnishes an efficient means for cooling anintegrated circuit and does so without the need for larger heatspreaders which not only add weight but also do not transfer heat awayfrom the integrated circuit as efficiently as does the heat pipe of theinvention.

It is to be understood that the form of this invention as shown ismerely a preferred embodiment. Various changes may be made in thefunction and arrangement of parts; equivalent means may be substitutedfor those illustrated and described; and certain features may be usedindependently from others without departing from the spirit and scope ofthe invention as defined in the following claims. For example, throughholes could also penetrate heat pipe boundary structures with curvedsurfaces or heat pipe boundary structures with offset planes whichcreate several different levels for contact with heat sources or heatsinks.

1-7. (Cancelled)
 8. A heat pipe for spreading heat comprising: aboundary structure including spaced-apart first and second plates thatdefine an enclosed vapor chamber; a depression formed in said firstplate which projects into said vapor chamber and is bonded to saidsecond plate; and an opening defined through said depression and saidsecond plate wherein said opening is isolated from said vapor chamber.9. A heat pipe for spreading heat according to claim 8 comprising atleast one spacer positioned within said vapor chamber and extendingbetween and contacting said first and second plates.
 10. A heat pipe forspreading heat according to claim 8 wherein said spaced-apart first andsecond plates include confronting interior surfaces; and a wickpositioned upon said confronting interior surfaces including thatportion of the interior surface of said first plate that forms a surfaceof said depression within said vapor chamber.
 11. A heat pipe forspreading heat according to claim 10 wherein said wick is constructedwith at least two separate sections of different materials, with onesection being located on said first plate interior surface and beingformed of a material with higher heat conductivity than sections locatedon said second plate interior surface.
 12. A heat pipe for spreadingheat according to claim 8 wherein said depression comprises an annularouter surface that is bonded to a corresponding annular edge surface insaid second plate.
 13. A heat pipe for spreading heat according to claim8 wherein said first and second plates each include a peripheral liplocated at an edge of said boundary structure which are bonded together.14. A heat pipe for spreading heat comprising: a boundary structureincluding spaced-apart first and second plates that define an enclosedvapor chamber; a depression formed in said first plate which projectsinto said vapor chamber and is bonded to said second plate; an openingdefined through said first plate depression and said second platewherein said opening is isolated from said vapor chamber; and adepression formed in said second plate which projects into said vaporchamber and is bonded to said first plate.
 15. A heat pipe for spreadingheat according to claim 14 wherein said depression formed in said secondplate comprises a flat portion that is in contact with an inner surfaceof said first plate.
 16. A heat pipe for spreading heat according toclaim 14 wherein said spaced-apart first and second plates includeconfronting interior surfaces; and a wick positioned upon saidconfronting interior surfaces including that portion of the interiorsurface of said first plate that forms a surface of said depressionwithin said vapor chamber.
 17. A heat pipe for spreading heatcomprising: a boundary structure including spaced-apart first and secondplates that define an enclosed vapor chamber; at least one hollow columnpositioned within said vapor chamber and sealingly bonded to said firstand second plates, having an open first end that opens through saidfirst plate and an open second end that opens through said second plateso as to form at least one mounting hole that is isolated from saidvapor chamber.
 18. A heat pipe for spreading heat according to claim 17wherein said spaced-apart first and second plates include confrontinginterior surfaces; and a wick positioned upon said confronting interiorsurfaces of said first and second plates the exterior surface of said atleast one hollow column disposed within said vapor chamber.
 19. A heatpipe for spreading heat comprising: a boundary structure including afirst plate and a second plate arranged in spaced apart relation, eachof said plates including interior confronting surfaces and a peripherallip located at an edge of said boundary structure which are bondedtogether so as to define an enclosed vapor chamber; a depression formedin said first plate which projects into said vapor chamber, is spacedfrom said peripheral lip, and is bonded to said second plate; an openingdefined through said first plate depression and said second platewherein said depression comprises an annular outer surface that isbonded to a corresponding annular edge surface in said second plate andfurther wherein said opening is isolated from said vapor chamber; atleast one spacer extending between and contacting said first and secondplates; and a wick positioned upon said confronting interior surfacesincluding that portion of the interior surface of said first plate thatforms a surface of said depression within said vapor chamber.
 20. A heatpipe for spreading heat comprising: a first plate having acircumferential edge lip bounding an inner surface and at least onehollow column that is integral with said first plate and which projectsoutwardly relative to said inner surface; a second plate arranged inspaced apart confronting relation to said first plate and including acircumferential edge lip bounding an inner surface and at least oneopening through said second plate, said edge lips of said first andsecond plates being bonded together so as to define a vapor chamber;wherein a hollow column bonded at one end to said second plate so as tocoaxially align said hollow column with one of said at least oneopenings in said second plate thereby to form a mounting hole thatextends through said first plate and said second plate and is isolatedfrom said vapor chamber.
 21. A heat pipe for spreading heat comprising:a first plate having a circumferential edge lip bounding an innersurface and a depression which projects outwardly relative to said innersurface; a second plate arranged in spaced apart confronting relation tosaid first plate and including a circumferential edge lip bounding aninner surface and at least one opening through said second plate, saidedge lips of said first and second plates being bonded together so as todefine a vapor chamber; wherein said depression having an open endedtubular cross-section and an outer surface, a portion of which outersurface is bonded to said second plate so as to coaxially align saiddepression with one of said at least one openings in said second platethereby to form a mounting hole that extends through said first platedepression and said second plate and is isolated from said vaporchamber.
 22. A heat pipe for spreading heat comprising: a first platehaving a circumferential edge lip bounding an inner surface and a hollowcolumn which projects outwardly relative to said inner surface; a secondplate arranged in spaced apart confronting relation to said first plateand including (i) a circumferential edge lip bounding an inner surface,(ii) at least one depression which projects into said vapor chamber andthat is bonded to said inner surface of said first plate, and (iii) atleast one opening through said second plate, said edge lips of saidfirst and second plates being bonded together so as to define a vaporchamber; wherein said hollow column in said first plate opens at a firstend and a second end and is bonded to said second plate at said secondend so as to coaxially align said hollow column with said at least oneopening in said second plate thereby to form a mounting hole thatextends through said first plate depression and said second plate and isisolated from said vapor chamber.
 23. An integrated circuit chip coolingstructure comprising: a top plate having an edge lip that bounds a topsurface and a bottom surface; a bottom plate having an edge lip thatbounds a top surface and a bottom surface, said edge lips of said topand bottom plates being bonded together so as to define a vapor chamber;a capillary wick provided within said vapor chamber; wherein said topplate and said bottom plate each define at least one opening such thatthe at least one opening of said top plate and the at least one openingof said bottom plate are vertically aligned; at least one hollow columnextending between said top plate and said bottom plate, said at leastone hollow column having a top end sealed to a periphery of said openingdefined in said top plate and a bottom end sealed to a periphery of saidopening defined in said bottom plate, said at least one hollow columnhaving a central passageway sized to receive a fastener.
 24. Theintegrated circuit chip cooling structure according to claim 23comprising at least one heat sink, said at least one heat sink having abottom surface attached to the top surface of said top plate and atleast one mounting hole communicating with a hollow column; and afastener extending through said hollow column and through said mountinghole of said at least one heat sink to secure said heat sink to said topand bottom plates.
 25. The integrated circuit chip cooling structureaccording to claim 23 wherein said at least one hollow column each havea top shoulder disposed near a top end and bonded to a bottom surface ofsaid top plate around one opening.
 26. The integrated circuit chipcooling structure according to claim 25 wherein said at least one hollowcolumn each have a bottom end downwardly extending out of said bottomplate through one opening defined in said bottom plate.
 27. Theintegrated circuit chip cooling structure according to claim 24 whereinsaid at least one heat sink comprises a base and a plurality of finslocated on one side of said base.
 28. The integrated circuit chipcooling structure according to claim 23 wherein said at least one hollowcolumn has a top shoulder disposed near a top end and bonded to a bottomsurface of said top plate around one opening defined in said top plateand a bottom shoulder disposed near a bottom end and bonded to a topsurface of said bottom plate around one opening defined in said bottomplate.
 29. The integrated circuit chip cooling structure according toclaim 23 wherein said capillary wick comprises an upper plate-like wicklocated on a bottom surface of said top plate, a bottom plate-like wicklocated on a top surface of said bottom plate, and a plurality ofvertical wick elements provided between said upper plate-like wick andsaid bottom plate-like wick, said vertical wicks each having a first endintegral with one of said top and bottom plate and a second endsupported on the other of said top and bottom plates; said at least onehollow column being inserted through said top plate-like wick and saidbottom plate-like wick.